1. Field of the Invention
The invention relates to a continuous process for preparing SiOC-containing compounds.
2. Background Art
The preparation of SiOC-containing compounds by means of column technology, in particular alkoxysilanes and alkoxy-rich silicone resins, has been known for some time. Starting from chlorosilane(s), alkoxylation takes place in a column and its bottom. In the case of alkoxy-rich silicone resins, water is metered in a controlled manner in deficiency based on the SiCl units metered in.
U.S. Pat. No. 3,792,071 A1 describes a process for continuously preparing alkoxysilanes or alkoxypolysiloxanes, in which, in a column with reflux condenser, chlorosilane is fed in at the top, gaseous alcohol fed to the lower third, and water at any point, the column temperature being at least 0.5° C. above the boiling point of the alcohol. The material flowing out of the lower end of the column is freed of alcohol in a circulating evaporator, and the alcohol then recycled into the column.
As a further development of this process, U.S. Pat. No. 4,298,753 A1 describes a process for continuously preparing alkoxysilanes or alkoxypolysiloxanes, in which, in a first reactor, for example, a stirred tank or stirred reactor, including U-shaped or annular reactors, with reflux condenser, chlorosilane, 0.5 to 0.9 eq. of alcohol/Si—Cl, and optionally water is introduced concurrently in liquid form and mixed, the effluent liquid being fed into the top of a column used as a 2nd reactor. Gaseous alcohol for the reaction of the remaining SiCl groups is metered into the lower third of the column, optionally in addition to water at any point, and the product is removed below the alcohol feed.
A further process for continuously preparing monomeric or oligomeric alkoxysilanes is described by U.S. Pat. No. 4,506,087 A1. In a reactor with an attached column, chlorosilane and optionally water are metered in liquid form into the reactor, and alcohol is metered in gaseous form into the lower section of the column. Condensed alcohol leaving the column at the top is introduced into the reactor. The product leaving the reactor is fed to the top of the column and the end product is withdrawn at the lower end of the column.
U.S. Pat. No. 6,767,982 B2 discloses a further process for the continuous preparation of a mixture of organoalkoxysiloxanes. In a first stage (stirred tank or reaction column), at least one organotrichlorosilane is reacted with water (0.59 to 0.95 mol based on silane) and alcohol (0.5 to 100 mol based on silane) at a temperature of from 0 to 150° C., HCl is removed simultaneously, and the crude product, after a delay time of from 0.5 to 180 min, is transferred partly into a reaction distillation column of the second process stage. Alcohol is fed into the lower section of this column and volatile components are removed as the overhead product. The desired organopolysiloxanes are obtained at a temperature of from 50 to 200° C. in the bottoms and are freed of residual chloride in an aftertreatment step. The volatile components removed as the top product are, after condensation, recycled into the first reaction unit.
It is evident from the prior art that a continuous process utilizing at least one column unit is particularly advantageous for preparing SiOC-containing compounds. However, the productivity of these processes is restricted by the limited throughput of each column unit, and the throughput of the column in which the majority of the HCl is formed limits the entire process. In this column, a stream of a liquid phase (alkoxysilane/part-alkoxylate formed) is present from the top downward and, at the same time, a stream of a gas phase (HCl) from the point of formation at the top of the column. In the event of too strong a stream of the gas phase, a pressure is generated which is so great that the mobile liquid phase no longer moves downward owing to gravity but rather is discharged at the top of the column.